The growth in optical communications has been fueled by the extraordinary bandwidth which is available on optical fiber. Such bandwidth enables thousands of telephone conversations and television chapels to be transmitted simultaneously over a hair-thin fiber that is generally made from a high-quality glass material. Light travels primarily within the core region of the fiber because the core has a slightly higher index of refraction than the surrounding region. And while optical transmission has significant advantages vis a vis metallic wire lines, optical fibers do have loss and do not have infinite bandwidth.
Insofar as loss is concerned, staggering advances have been made in the quality of the glass material (nearly pure silica--SiO.sub.2) used in making optical fibers. In 1970, an acceptable loss for glass fiber was in the range of 20 dB/km; whereas today, losses in the range 0.22-0.25 dB/km are routine. Indeed, the theoretical minimum loss for glass fiber is about 0.16 dB/km, and it occurs at a wavelength of about 1550 nanometers (nm).
Various mechanisms limit a fiber's bandwidth. In multimode fiber, for example, there is modal dispersion in which pulses of light that enter one end of the fiber are spread as they emerge from the other end of the fiber. This is because multimode fiber supports hundreds of different modes (paths) of a particular wavelength. And when the different modes are combined at the other end of the fiber, the net result is pulse spreading. However, a fiber can be designed to support only the fundamental mode of a particular wavelength and is referred to as a singlemode fiber. Such a fiber has an extremely high bandwidth. But even here, a pulse of light that is introduced into one end of a singlemode fiber is somewhat spread as it emerges from the other end. This is because the act of turning a light source of a single wavelength on and off (i.e., a light pulse) generates a large number of harmonically related wavelengths, and different wavelengths travel through glass at different speeds. Accordingly, light pulses that are injected into one end of a glass fiber spread out as they arrive at the other end because the different wavelengths (colors) arrive at different times. Not surprisingly, this is referred to as chromatic dispersion, and is the optical counterpart to that which electrical engineers call delay distortion.
As shown in FIG. 1, a light pulse having an 800 nm wavelength arrives about 10 nanoseconds after one having a 900 nm wavelength in a typical glass fiber. A common way of expressing the chromatic-dispersion properties of a fiber is to take the derivative of the delay curve in FIG. 1 with respect to wavelength. This derivative is merely the slope of the delay curve as a function of wavelength and is referred to as chromatic dispersion (D), which is graphically shown in FIG. 2. The composition of glass that is generally used in making optical fiber has zero dispersion at a wavelength .lambda..sub.0 in the region of 1310 nm. But, as noted above, the theoretical minimum loss for a glass fiber is in the region of 1550 nm. Interestingly, nature appears to smile benignly upon optical transmission in this wavelength region since it is where the only practical fiber amplifier operates. (Erbium-doped fiber is used to amplify optical signals having wavelengths in the 1530-1565 nm region where there is a transition in the Er.sup.3+ dopant ion.)
It has been learned that a singlemode fiber can be designed to have its zero dispersion wavelength .lambda..sub.0 anywhere generally in the 1300-1700 nm region by proper control of dopant, doping concentration, core diameter, and refractive-index profile. Because of the desirability of operating in the 1550 nm region, singlemode fibers have been designed having a zero dispersion wavelength .lambda..sub.0 at about 1550 nm. Such fibers have become exceedingly popular and are generally referred to as dispersion shifted fibers (DSF).
Data transmission rates over an optical fiber can be increased via Wave Division Multiplexing (WDM) in which several channels are multiplexed onto a single fiber--each channel operating at a different wavelength. Using already-installed, non-shifted fiber, it has been demonstrated that by transmitting four channels in the 1550 nm region, the channels being separated by about 1.6 nm, capacity may be increased four-fold over single channel operation to 4.times.2.5 Gb/s=10 Gb/s (1 Gb/s=1 billion bits per second). However, it has been found that 4-channel WDM operation is essentially precluded by the use of DSF, and so DSF which is already in place is found to be limited either to single channel operation or to WDM systems which have limited span lengths, fewer channels, or lower bit rates per channel.
U.S. Pat. No. 5,327,516 (the '516 patent) discloses an improved optical fiber, designated WDM fiber, which is particularly effective for the transmission of multiple channels of information--each operating at a different wavelength. Such fiber is commercially available from Lucent Technologies Inc. as its TrueWave.RTM. optical fiber, and is capable of supporting at least eight channels separated from one another by 0.8 nm over span lengths greater than 360 km without regenerators. And Lucent's 1450D Dense Wave Division Multiplexer (DWDM) enables eight channels, each carrying 2.5 Gb/s of information, to be routed to/from the TrueWave optical fiber. At that rate, a system is able to transmit the equivalent of almost 5,000 novels in one second--about eight times as much as most long-distance fiber-optic systems. Indeed, by increasing the data rate of the individual channels to 20 Gb/s, increasing the number of channels to 25, and transmitting at two different polarizations, the transmission of one terabit per second (1 Tb/s=1000 Gb/s) has already been demonstrated over 55 kilometers of TrueWave optical fiber. Heretofore, such a speed has been reverently referred to as the "Holy Grail" of data transmission.
Briefly, the '516 patent reduces non-linear interaction between channels by introducing a small but critical amount of positive or negative chromatic dispersion at 1550 nm. Such non-linear interaction is known as four-photon mixing, and it severely restricts system design as well as operating characteristics. And while the use of WDM fiber is highly desirable, a dilemma is created. Whereas the introduction of dispersion is desirable for the purpose of minimizing four-photon mixing, it is undesirable because it causes pulse spreading as discussed above.
A number of patents have already dealt with the problem of compensating dispersion including U.S. Pat. No. 4,261,639 (Kogelnik et al.); U.S. Pat. No. 4,969,710 (Tick et al.); U.S. Pat. No. 5,191,631 (Rosenberg); and U.S. Pat. No. 5,430,822 (Shigematsu et al.). These patents compensate dispersion by inserting modules at appropriate intervals. The modules usually contain Dispersion-Compensating Fiber (DCF) of an appropriate length to produce a dispersion of approximate equal magnitude (but opposite sign) to that of the transmission fiber in the route. Unfortunately, these modules consume space, introduce extraneous loss, and increase cost.
Proposals have previously been made to construct cables in which all fibers in the cable are of one type--either positive or negative dispersion. Cables containing fibers of one type would be spliced at appropriate intervals to cables containing fibers of the other type. This approach has the disadvantage of requiring that two types of cables be manufactured and stored in inventory. Additionally, major problems arise with the administration of both types of cables during construction, keeping accurate records of where each type of cable is used and stored, and performing routine maintenance.
What is needed, and what the prior art appears to be lacking, is an optical cable which jointly enables the reduction of four-photon mixing and cumulative dispersion without the use of DCF modules.